Such apparatuses include, for example, the one disclosed in Japanese Patent Application Kokai No. 60-148124, which is constituted by a furnace core tube for accommodating the wafers, a heating unit assembled of a heater member surrounding the tube and of an insulating heat-resistant cylindrical member made of an adiabatic material for supporting the heater member, and an outer cylinder for accommodating therein the furnace core tube and the heating unit. On the interior surface of the cylindrical member, there is formed a helical recess in which the heater member is wound and buried. On opposite ends of the outer cylinder, there are fixed adiabatic cylindrical supporting members for supporting both end portions of the furnace core tube so that the central axis of the tube and the winding central axis of the heater member can substantially coincide with each other.
In the above type of conventional apparatus, heat loss resulting from heat dissipation can be prevented due to the buried arrangement of the heater member in the interior surface of the adiabatic cylindrical member. However, problems are still encountered therewith. Specifically, no efficient use of the radiation heat from the heater member can be achieved, and an adiabatic cylindrical member of the heating unit having a large heat capacity requires a long time to obtain the desired temperature rise and drop during operation of the furnace core tube. Furthermore, deterioration of the heater member may result from heat accumulation in the burying recesses of the cylindrical member, which will lead to a a burnout of the heater member. In the event of such burnout, it is difficult to replace just the heater member alone since it is deposited in the recesses, and accordingly the entire heating unit usually must be exchanged as a whole. A problem arises in this case in that the apparatus may suffer from the production of many contamination particles at the time of the exchange because the adiabatic cylindrical member and the cylindrical supporting members are made of a material such as hardened ceramic fibers or the like which are liable to yield particles, if rubbed against the furnace core tube.
Moreover, since the furnace core tube is supported at its opposite end portions by the cylindrical supporting members, the tube is subjected to the intense bending or distorting force acting on its central portion by its own weight and also that of the wafers being treated disposed in the tube, with the result that maintaining coincidence of the central axis of the tube and the winding central axis of the heater member is very difficult. Consequently, the temperaure distribution in the furnace core tube is adversely changed, or it becomes difficult to extract the tube from the within of the outer cylinder.